In a known fingerprint detection apparatus, the finger under investigation is usually pressed against a flat surface, such as a side of a prism or a glass plate, and the ridge and valley pattern of the finger tip is illuminated with an interrogating light beam. In the optical devices which employ prisms, the prism has a first surface upon which a finger is placed, a second fingerprint viewing surface disposed at an acute angle to the first surface, and a third illumination surface through which light is directed into the prism. The incident beam of light is reflected from the first surface and exits through the fingerprint viewing surface. An image producing lens or lens system is provided for receiving the beam reflected from the valleys of the subject fingerprint and for producing an image of the subject fingerprint at an image sensor, e.g. a charge coupled device (CCD) or the like for converting the fingerprint image into an electric signal. An amplifying/analysing signal processing circuit and a monitor for displaying the fingerprint image are also provided. In some cases, the illumination surface is at an acute angle to the first surface, as seen for example, in U.S. Pat. Nos. 5,187,482 and 5,187,748. In other cases, the illumination surface is parallel to the first surface, as seen for example in U.S. Pat. Nos. 4,924,085, 5,109,427 and 5,233,404.
Fingerprint identification devices of this nature are generally used to control building-access or information-access of individuals to buildings, rooms, and devices such as computer terminals.
One of the problems associated with fingerprint sensors concerns the reliable and accurate transformation of ridge and valley pattern of the finger tip into electrical or optical signals to be stored in a digital format. Optical systems as described above, for example using a prism, require sophisticated equipment and tend to be bulky and costly.
It is extremely difficult to produce a single large image sensor, for example comprised of a single piece of silicon (called a chip or a die) cut from a silicon wafer. Fabricating a device with a large area is impractical due to the cost of manufacture and lower manufacturing yields for larger dies than for smaller dies. When square or rectangular dies are cut from a substantially round silicon wafer, there is less loss at the edges of the wafer when small dies are cut than when large dies are cut. Faults within a die often render the die unusable. As an entire die is discarded, it is evident that smaller dies result in less wasted material. This is illustrated hereinbelow.
Furthermore, current, conventional photolithographic systems are typically equipped for the production of dies that have a maximum dimension of about 0.4 inches to 0.5 inches. In optical CCDs, larger die areas capture more light and provide higher resolution output. A higher yield due to smaller dies results in lower cost.